diff --git a/src/app.js b/src/app.js
new file mode 100644
index 0000000..0b3ce0c
--- /dev/null
+++ b/src/app.js
@@ -0,0 +1,13 @@
+function add(n1, n2, showResult, phrase) {
+    if (showResult) {
+        console.log(n1 + n2);
+    }
+    else {
+        return n1 + n2;
+    }
+}
+var number1 = 5;
+var number2 = 2.8;
+var printResult = true;
+var resultPhrase = 'Result is: ';
+add(number1, number2, printResult, resultPhrase);
diff --git a/src/app.ts b/src/app.ts
index 36f14ed..98000c7 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -1,6 +1,7 @@
 function add(n1: number, n2: number, showResult: boolean, phrase: string) {
+  const result = n1 + n2; //result는 number라고 추론을 한다.
   if (showResult) {
-    console.log(n1 + n2);
+    console.log(result + phrase);
   } else {
     return n1 + n2;
   }
@@ -10,5 +11,6 @@ const number1 = 5;
 const number2 = 2.8;
 const printResult = true;
 const resultPhrase = 'Result is: ';
+// resultPhrase = 0;  //error -> 타입 추론으로 resultPhrase는 string이다. 따라 0이라는 number타입을 할당할 수 없다.
 
 add(number1, number2, printResult, resultPhrase);
\ No newline at end of file